This invention is concerned with an offshore drilling rig, and more particularly with the type of offshore drilling rig which is to be supported on the ocean bottom.
Offshore drilling rigs are huge structures which, as their name indicates, are located off shore in the ocean and are in most cases employed to drill for oil in the continental shelf. Certain of these rigs are of the type that float; others are of the type having legs which extend through the water to the ocean bottom and are there anchored in suitable manner; at the top the legs carry a working platform that is elevated above the level of the ocean. The present invention is concerned with the latter type of drilling rig.
It is known from the prior art (compare "Steel Times," December, 1973, page 859) to form an underwater platform on the ocean bottom, and to mount the legs of the drilling rig on this platform. In order to assure that the platform will be properly and uniformly supported and not be subject to undesirable stresses, it is necessary to grade the ocean bottom before the platform can be installed.
Another prior-art proposal suggests to drive bundles of timber or steel girders into the ocean bottom around the lower ends of the supporting legs of the rig; the timber or the girders may be driven to a depth of up to 90 meters. Of course, to drive these elements into the ocean bottom, it is necessary to carry out extensive underwater operations, which are expensive.
Another problem encountered in connection with the prior art is the manner in which the drilling rig is moved to its ultimate place of use. It is conventional (compare the periodical "Meerestechnik," Germany, 1973, No. 6, pages 207 ff) to assemble the framework structure of such rigs, including the legs, in a horizontal position in dry docks or wharves; upon completion of the assembly they are then towed in horizontal position to the installation site where they are moved to upright position and anchored. The towing and the change from horizontal to upright position requires special flotation devices and is complicated.